Canister-type vacuum cleaners consist of a suction unit which is generally stationary during use of the cleaner. In addition to an electric motor for creating a suction force, the unit also includes a dust bag for filtering dust particles out of air drawn into the unit.
During use, the suction unit is attached to a head by means of an air hose. It is the head, which may be equipped with a rotatable brush, which draws dirt from the carpet or other object being cleaned and passes it through the hose to the suction unit where dirt particles are filtered from the air stream.
Most canister-type vacuum cleaners do not include a means for determining when the dust bag is full. As the dust bag gradually fills during use, the dust particles pile up to reduce the suction and thus the effectiveness of the cleaner. This reduced effectiveness occurs so slowly that the user of the cleaner is not aware of the reduction until a point is reached where the cleaner picks up so little dirt as to make it obvious that something is wrong. As a result the cleaner may be used for some time at a low level of effectiveness before it occurs to the user that the bag needs changing or to be emptied.